The present invention relates to solution coating barrier layers on plastic films.
The gas permeability of certain plastic articles, such as films, can be reduced by coating the article with a barrier material. Ethylene/vinyl alcohol copolymer (EVOH) is recognized as a barrier material which is especially good for reducing the permeability to oxygen. By coating plastic with an EVOH layer it is possible to make clear, strong film which is well suited for packaging oxygen-sensitive products such as food and chemicals.
One of many methods to apply polymer coatings to substrates is solution coating. Typically in such process, the coating material is dissolved in a convenient solvent, the solution is deposited on the film surface and thereafter the solvent is removed. Solution coating is advantageous for placing very thin coatings on sheets, films and substrates which have complex shapes such as containers. Generally, alcohols, and mixtures of alcohols with water, are preferred solvents for solution coating substrates with EVOH because they are inexpensive and have moderately low toxicity. Solutions of EVOH in alcohols, however, tend to gel or precipitate after short-term aging. Consequently, they have only limited shelf life. Furthermore, alcohol-based solutions of EVOH have high solution viscosities. Dilute solutions must be used in order to attain sufficiently low solution viscosity for proper coating process operation. Dilute solutions, however, are costly because of the large amount of solvent consumed and long drying time needed to remove the solvent. Use of dilute solutions is also environmentally unattractive because of the need to recover large quantities of spent solvent.